1. Field of the Invention
The present invention relates to a power converter, and more particularly, to the control circuit of the power converter.
2. Description of Related Art
FIG. 1 shows a conventional power converter. The power converter comprises a bridge rectifier 35, capacitors 30 and 45, a power transistor 20, resistors 25, 31 and 32, a rectifier 40, a controller 100, and a transformer 10 with a primary-side and a secondary-side. The primary-side includes a primary winding NP and an auxiliary winding NA. The secondary-side includes a secondary winding NS. One terminal of the capacitor 30 is coupled to an output of the bridge rectifier 35 and one terminal of the primary winding NP. The other terminal of the capacitor 30 is coupled to a ground. The bridge rectifier 35 rectifies an AC input voltage VAC to a DC input voltage VIN at the capacitor 30, and the DC input voltage VIN is supplied to the primary winding NP.
A drain terminal of the power transistor 20 is connected to the other terminal of the primary winding NP in series. The power transistor 20 is used to switch the transformer 10 and control transferring of power energy from the primary winding NP of the transformer 10 to the auxiliary winding NA and the secondary winding NS of the transformer 10. The controller 100 is coupled to a gate terminal of the power transistor 20. A switching signal SW is generated by the controller 100. The switching signal SW is supplied to the power transistor 20 and controls the power transistor 20 to switch the transformer 10. In other words, the controller 100 generates the switching signal SW coupled to switch the transformer 10 via the power transistor 20. The resistor 25 is coupled between a source terminal of the power transistor 20 and the ground. A switching current IP of the transformer 10 flows through the transistor 20 that generates a current-sense signal VCS at the resistor 25. The current-sense signal VCS is supplied to the controller 100.
The rectifier 40 and the capacitor 45 are coupled to the secondary winding NS of the transformer 10 for generating an output voltage VO and an output current IO1 of the power converter. The resistors 31 and 32 are connected in series. The resistors 31 and 32 are coupled from the auxiliary winding NA of the transformer 10 to the ground for detecting the output voltage VO and generating a signal VS during the switching of the transformer 10. The signal VS is generated at a joint of the resistors 31 and 32. An input terminal VS of the controller 100 is coupled to the joint and receives the signal VS. The signal VS is correlated to the output voltage VO and is related to the transformer's demagnetizing time. The demagnetizing time of the transformer 10 is used for controlling the output current IO1. The switching signal SW is generated in accordance with the signal VS (the reflected voltage of the transformer 10) for regulating the output (output voltage VO and/or the output current IO1) of the power converter.
In order to control the output current IO1, for example providing a constant output current for the battery charge or LED lighting, etc., it requires developing a current-feedback loop for the regulation. The power converter normally includes the output voltage and/or the output current regulation. Refer to the skill of the output current regulation, it had been disclosed in a prior art “Control circuit for controlling output current at the primary side of a power converter”, U.S. Pat. No. 6,977,824. The detail of the voltage-loop and the current-loop operation can be found in the prior art of “Close-loop PWM controller for primary-side controlled power converters”, U.S. Pat. No. 7,016,204.
For achieving a stable feedback loop, the current-feedback loop is compensated to a low bandwidth in general. Thus, a higher overshoot current would be produced during the load change, particularly when the load is changed from the light load to the heavy load, as shown in FIG. 2.
The drawback of the conventional power converter is the slow response of the current-feedback circuit. In order to achieve the loop stability, the bandwidth of frequency compensation is low. Thus, its loop response to the load changing is slow. FIG. 2 shows an output current waveform that is controlled by the controller 100 of FIG. 1. When the load (Load) is changed, the output current IO1 includes the higher overshoot current within a TD period. The TD period is related to the loop response of the current-feedback circuit.